It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. It could be a failing fuel pump, a restricted catalytic converter, a failing transmission, basic tests need to be done to get to the root of the problem. Is the Check Engine Light on?
This could be a possibility, it could be a OXYGEN SENSOR, with regulates how much throttle that gets delivered to the motor through the computer system. So for example, u can be going 30 mph when u are really going 10 mph. Hope this helps.